For semiconductor devices (generally silicon devices), more specifically, a gate oxide film MOS transistor, an oxide film MOS capacitor or the like; a silicon dioxide film (hereinafter oxide film is used for abbreviation) is used. These oxide films need to have a high dielectric breakdown voltage and a high charge to (dielectric) breakdown. Clearing the wafer is one of the very important processes in manufacturing an oxide film. It is required that the wafer be cleaned and have a high quality, that is, a low fixed charge density, and a low interface-state density. On the other hand, as devices become fine and highly-integrated, gate oxide films or capacitance oxide films are becoming thinner. For example, according to the design rule of 0.1 .mu.m or less, an ultra thin gate oxide film having a thickness of not more than 4 nm is required. Conventionally, a gate oxide film of an MOS transistor has been formed by the method in which semiconductor substrate is exposed in an oxidizing atmosphere, for example, in an atmosphere of dry oxygen or in an atmosphere of steam, at a temperature of 600.degree. C. or higher (See, for example, VLSI technology, S. M. Sze edition (1983) 131-168 page).
Moreover, other than thermal oxidation, the chemical vapor deposition method (CVD method) is employed, in which mono-silane is thermally decomposed to deposit on the surface of the substrate. Methods of growing an oxide film at a low temperature include the method of forming a chemical oxide film by soaking the semiconductor substrate in a chemical for promoting oxidation, such as nitric acid; and the method of forming an oxide film by anodic oxidation. However, in the case of chemical oxide films, the growth of film thickness is limited. On the other hand, in the case of anodic oxidation, the range of controllability of film thickness is relatively wide, but the electric characteristics such as, the interface characteristics, dielectric breakdown characteristics, or the like are not satisfactory. Other methods of forming oxide films at low temperature include the method of forming an oxide film by conducting a thermal oxidation by ultraviolet irradiation and the method of forming an oxide film by oxidizing in plasma. In the conventional methods mentioned above, it has been difficult to form high-quality thin films having high controllability and high reproducibility.
Moreover, thermal oxidation at relatively high temperature has the problem of a lack in controllability of (the film thickness during formation of an oxide film having a thickness of 4 nm or less. If oxidation is conducted at a low temperature in order to improve the controllability of the film thickness, there arise problems in the quality of the formed oxide film, that is, the interface-state density is high, the fixed charge density is high or the like. Moreover, an oxide film deposited by the CVD method has the same problems in the controllability and the quality of film. In particular, occurrence of an interface-state density not only deteriorates the hot-carrier immunities of a transistor, but also causes instability in the threshold voltage of the transistor and deterioration in mobility of carriers. In a case where a small device is employed, this may be a fatal problem. Moreover, it is required to decrease the heat treating process by making the element small. In particular, from the viewpoint of flexibility in designing devices and process, in the conventional method of forming a gate oxide film by the use of a thermal oxide film at a relatively high temperature, the gate oxide film needs to be formed before the metallic interconnection process is carried out. So far, in order to obtain a low resistance, aluminum or aluminum alloy has been used for the metallic interconnection. Since the melting point of aluminum is 660.degree. C. and, moreover, hillocks (abnormal protrusions on the surface of an aluminum wiring taking place during heat treatment) may be occur, the heat treatment after metallic wiring is required to be conducted at a temperature not higher than 400.degree. C. Therefore, in the case where the conventional thermal oxidation method is employed, it is difficult to form a gate oxide film after the metallic interconnection process. Moreover, in the case where an oxide film is formed by the heat treatment at a temperature of not higher than 400.degree. C. for approximately one hour, the film thickness turns out to be not more than 1 nm, thus making it difficult to form a thin film usable for a gate oxide film.